The present invention relates to model-based control of a gas turbine engine.
Modern Brayton and Ericsson cycle engines, including gas turbine engines for aircraft applications, continue to grow more complex. These engines require sophisticated control systems to handle increasing operational demands at reduced tolerances. Such engine control systems command engine actuators for control parameters such as fuel flow rate and variable engine geometries to achieve desired values of output parameters such as net thrust or engine rotor speed. A variety of control methods are currently used toward this end, including model-based control algorithms using predictive models that relate thermodynamic parameters such as flow rate, pressure, and temperature to input and output variables such as overall thrust, power output, or rotational energy.
Engine control systems are typically provided with a plurality of inputs including both current operating parameters and target parameters. Current operating parameters may include engine parameters such as rotor speeds, engine temperatures, and flow rates, as well as environmental parameters such as altitude and environmental air pressure and flow rate. Some current operating parameters are directly measured, while others may be fixed at manufacture or estimated based on measured parameters. Target parameters may include desired rotor speeds or net thrust values specified according to desired aircraft activities.
In addition to achieving specified target parameters, engine control systems are expected to avoid engine trajectories resulting in engine states that unduly reduce component lifetimes or increase likelihoods of undesired events such as engine surge, compressor stall, or engine blowout. These constraints may take the form of additional inputs provided during engine operation, maintenance, or installation. Engine control systems may be expected to achieve target parameter values while remaining within specified allowable engine trajectory ranges, or while avoiding forbidden engine trajectory ranges, or any combination of the two.
Some engine control systems rely on component-based mathematical engine models (see, e.g. U.S. Pat. No. 8,131,384). Such systems have previously used pre-calculated invertible linearizations of the component-based model at several preselected steady state conditions corresponding to possible engine operating states. During operation, the control system identifies the preselected steady state condition which most closely corresponds to current engine operating parameters. Linear coefficients of the corresponding component-based model approximation are then retrieved (e.g. from a lookup table), and used to solve for control parameters as a function of target parameters. Approximating current engine states by a set of preselected steady states is readily implemented in real time, but can give rise to inaccuracies which result in reduced control precision and diminished operating efficiency.